A typical fuel control system for use in controlling the supply of fuel to an aircraft engine comprises a metering valve operable to control the rate at which fuel passes from a supply line to a delivery line. A pressure drop control arrangement, for example comprising a pressure drop control valve and an associated spill valve, is operable to maintain a substantially constant pressure drop across the metering valve. A pressure raising and shut-off valve (PRSOV) controls the passage of fuel from the delivery line to a burner manifold, the PRSOV serving, in use, to maintain a minimum fuel pressure in a part of the fuel control system upstream thereof, so as to ensure that any fuel pressure operated devices arranged to receive fuel under pressure from the fuel control system can operate correctly.
The fuel control system receives fuel under pressure from a pumping unit that is driven by, and so operates at a speed related to the operating speed of, the associated engine. There is a need to provide, within the fuel control system, a mechanism whereby thrust can be controlled in the event of an overthrust condition arising. The scenario where a fuel control (i.e. fuel metering valve) fails, causing an upward run-away of engine thrust, is referred to as “thrust control malfunction” or “overthrust”. The un-commanded increase in thrust on one engine leads to a thrust imbalance (or thrust asymmetry) on the aircraft. The default response of a typical existing fuel control to an upward runaway is to shut-down the engine in order to protect the engine from overspeed induced failure. More recent aircraft and engine design combinations have arisen for which engine shut-down is not an acceptable response to thrust control malfunction at all flight conditions. At some flight conditions (i.e. landing approach) the flight crew may not be able to safely control a thrust asymmetry in one direction whilst the thrust asymmetry rapidly reverses (due to engine shut-down) and acts in the opposite direction. A means is therefore required to bring upward run-away failures under control, by maintaining operation and thrust from the faulty engine, rather than initiating shut-down of that engine.
A number of thrust control management, commonly known as thrust control malfunction accommodation (TCMA), systems to perform this function are known. For example U.S. Pat. No. 6,745,556 describes a TCMA system in which a control valve is operable to relieve the fuel pressure applied to one end of a metering valve with the result that the metering valve moves to a low or minimum fuel delivery position to allow fuel supply to the associated engine burner to be reduced. One of the arrangements described in U.S. Pat. No. 7,137,242 uses ports provided on a staging valve to allow the pressure at the inlet of the metering valve to be reduced in the event of an overthrust condition being sensed. EP 2339147 A describes a TCMA device operating in conjunction with a pressure drop control arrangement for a metering valve. The device comprises a control servo-valve which ensures that engine control can be maintained in the event of an overthrust condition in which fuel is delivered to a burner manifold at a rate higher than is desired, and also provides a mechanism whereby sufficient fuel can be delivered to the burner manifold under windmill relight conditions at which the engine is rotating at a relatively low speed.